The invention relates to moving bed processes for the production of aromatic compounds, in particular for reforming. More particularly, it concerns the combustion step used during regeneration of a used catalyst and is intended to restore its initial catalytic performances.
The catalyst generally comprises a support (for example, formed from at least one refractory oxide, the support possibly also including one or more zeolites), at least one noble metal (preferably platinum), and preferably at least one promoter metal (for example tin or rhenium), at least one halogen and optionally one or more additional elements (such as alkalis, alkaline-earths, lanthanides, silicon, group IVB elements, non noble metals, group IIIA elements, etc.). Catalysts of this type contain platinum, for example, and at least one other metal deposited on a chlorinated alumina support. In general, such catalysts are used to convert naphthenic or paraffinic hydrocarbons, which can be transformed by dehydrocyclisation and/or dehydrogenation, for reforming or for the production of aromatic compounds (for example for the production of benzene, toluene, ortho-meta- or para-xylenes). Such hydrocarbons originate from fractionation of crude oil by distillation or other transformation processes.
Such catalysts have been widely described in the literature.
One way of increasing the yields of such reforming or aromatic compound production processes is to reduce the operating pressures at which the different reactions of importance are carried out. As an example, reforming reactions were carried out at 40 bars 30 years ago; 20 years ago, at 15 bars. Today, reforming reactors usually operate at pressures of less than 10 bars, in particular in the range 3 to 8 bars.
The improvement in desirable reactions due to a reduction in pressure is accompanied by more rapid deactivation of the catalyst by coking. Coke, a high molecular weight compound constituted essentially by carbon and hydrogen, is deposited on the active sites of the catalyst. The H/C molar ratio of the coke formed varies from about 0.3 to 1.0. The carbon and hydrogen atoms form condensed poly-aromatic structures with a variable degree of crystalline organisation, depending on the function and nature of the catalyst and the operating conditions of the reactors. While the selectivity of transformation of the hydrocarbons to coke is very low, the amounts of coke accumulated on the catalyst can be large. Typically, for fixed bed units, such amounts are in the range 2.0% to 20.0% to 25.5% by weight. For circulating bed units, such amounts are below 10.0% by weight.
Coke deposition, which is more rapid at low pressure, also requires more rapid regeneration of the catalyst. Current regeneration cycles have become as short as 2-3 days.
Commonly assigned European patent EP-A-0 378 482 discloses a continuous process for regenerating a reforming or aromatic compound production catalyst which can overcome the inherent disadvantages of shorter and shorter cycles. One of the regeneration steps is oxychlorination of the catalyst. The present invention is concerns this step.
In EP-A-0 378 482, the used catalyst slowly travels from top to bottom in a regeneration vessel where it meets, in succession, a first radial moving bed combustion zone, a second radial moving bed combustion zone, an axial moving bed oxychlorination zone and an axial moving bed calcining zone, and:
a) in the first combustion zone, the catalyst is treated at a pressure of 3 to 8 bars, substantially equal to that in the first reforming reactor, at a temperature in the range 350xc2x0 C. to 450xc2x0 C., using a combustion gas based on an inert gas circulating as a co-current to the catalyst, comprising 0.01% to 1% of oxygen by volume, the combustion gas originating from a zone for washing the gases from the combustion, oxychlorination and calcining steps;
b) in a second combustion zone, the catalyst arriving directly from the first combustion zone is treated at a pressure of 3 to 8 bars, substantially equal to that in the first reactor, at a temperature which is higher by at least 20xc2x0 C. than the temperature in the first combustion zone, in the presence of gases originating from the first combustion zone and in the presence of an inert makeup gas to which up to 20% by volume of oxygen is added so that the catalyst is in contact with a gas comprising 0.01% to 1% by volume of oxygen, the gases circulating as a co-current with tile catalyst. The catalyst is then sent to the oxychlorination zone.
In order to present the prior art more clearly, FIG. 1 shows a figure from EP-A-0 378 482 which shows the catalyst supplied via pipe (17a), stored in zone (20) then passing to the regeneration stage via pipes (9) into a first combustion zone (101) then into a second combustion zone (105). Combustion is carried out by injecting a gas charged with oxygen (0.01-1% by volume) and if necessary, a supplemental supply (104) of an inert gas charged with oxygen or air. The combustion step ensemble corresponds to part A of FIG. 1.
We have established that, during operation of this process, good knowledge of the combustion, and thus of its progress and its control, are guarantees of proper operation of the unit and good regeneration quality. We have thus sought to improve combustion monitoring and control. The process proposed in the present patent application can obtain this result by separate management of the gases in each zone, thus controlling the conditions in each combustion zone and preferably terminating combustion by monitoring and controlling combustion completion.
More precisely, the process of the invention is a process for regenerating a moving bed of catalyst for reforming or for aromatic hydrocarbon production, the catalyst comprising a support, at least one noble metal and at least one halogen, the process comprising a combustion step treating the catalyst in at least two successive combustion zones, the process being characterized in that:
each combustion zone is separated from the adjacent combustion zones so as to allow catalyst to pass and to prevent the passage of gas;
at least one oxygen-containing gas is introduced into each zone, and the gases produced are extracted from each zone;
the severity of the operating conditions in each zone increases in the direction of flow of the catalyst.
Preferably, at least a portion, preferably all, of the gas extracted from one combustion zone is sent to the next zone (in the direction of catalyst flow) with possible oxygen addition (air, for example).
In general, the operating conditions are rendered more severe by increasing the temperature and/or the oxygen content of the incoming gas. Preferably, for each zone the oxygen content in the incoming gas is in the range 0.01% to 2%, the temperature of the inlet gas is in the range 350-600xc2x0 C., the residence time of the catalyst in one zone is in the range 5 min. to 3 hours and the WHSV (hourly mass flow rate of gas/mass of catalyst in contact with the gas) is in the range 1-50 hxe2x88x921.
The combustion step advantageously ends with a final zone for controlling and monitoring combustion completion in which the oxygen consumption is approximately less than 10% of the oxygen entering that zone. The temperature is preferably substantially constant.
The monitoring and control zone is preferably located in the lower portion of the last combustion zone, thus after the fame front.
Further, a gas containing oxygen in an amount which is higher than that of the gas entering the upstream levels (in the direction of flow of the catalyst) is introduced into the control and monitoring zone.
Thus the present invention defines combustion in a plurality of zones (or stages), where each stage is characterized by a temperature in that stage, a temperature of the incoming oxygen-containing gas, an oxygen content of the incoming gas, a gas flow rate and a duration of exposure of the coked catalyst to these conditions, in order to obtain more efficient combustion.